SAR data are data obtained by emitting a microwave from a synthetic aperture radar loaded in a flying object such as an airplane and a satellite toward the earth's surface, and observing a wave reflected from the earth's surface. Unlike observation by visible light, observation by this method is not affected by clouds, and is available day and night. Therefore, observation by this method is widely used as a remote sensing technique.
As a technique applying the SAR data, there is an interferometric SAR technique, in which interference processing is performed on a pair of SAR data (a set of master data and slave data), which is obtained by observing a same observation point at different two times or in different two directions. It is possible to generate and use digital surface model (DSM) data from InSAR data to be generated by using this technique. Further, it is also possible to generate and use differential InSAR (DInSAR) data, which is obtained by removing terrain fringes from InSAR data, for analysis of crustal deformation at two times or the like.
Nowadays, a frequency at which a representative earth observation satellite such as LANDSAT observes a same point is about once per several weeks. In order to improve the low frequency of observations, there is an action that many small satellites loaded with a lightweight and inexpensive sensor are launched to increase the frequency of observations. When this action is realized, a frequency of launching small satellites loaded with an SAR sensor increases in the future, and it is anticipated that many pieces of SAR data are distributed resultantly.
When many pieces of SAR data are distributed, the number of choices on pairs of SAR data when InSAR data are generated increases. Therefore, it is conceivably possible to generate InSAR data with higher accuracy when it is possible to select an optimum pair of SAR data.
High-accuracy InSAR data are necessary in order to generate high-accuracy DSM data or DInSAR data. It is desirable to use a pair of SAR data, which is selected to minimize a vertical baseline length, in order to generate high-accuracy InSAR data. As illustrated in FIG. 10, a vertical baseline length is a length of a line perpendicular to a line connecting a position of a satellite at which slave data are observed and an observation point, drawn from a position of a satellite at which master data are observed. As the vertical baseline length increases, it is less likely that SAR data may interfere. It is not possible to generate accurate InSAR data in an area where SAR data do not interfere. In view of the above, it is desirable to use a pair of SAR data that is selected to minimize the vertical baseline length.
Further, it is desirable to change a pair of SAR data, depending on a purpose of generation when InSAR data are generated.
For instance, in a case where InSAR data are generated for the purpose of generating DSM data, it is desirable to use a pair of SAR data that satisfies the following conditions:                An observation date/time of master data is near an expected date/time; and        An interval between observation dates/times of master data and slave data is small.        
The second condition is a condition for use in minimizing an area where it is not possible to generate accurate InSAR data. As the interval between observation dates/times of a pair of SAR data increases, a change in the earth's surface condition increases. When a change in the earth's surface condition increases, the pair of SAR data cannot interfere with each other, and an area where it is not possible to generate accurate InSAR data increases. In view of the above, it is desirable to make the interval between observation dates/times of master data and slave data small.
On the other hand, in a case where InSAR data are generated for the purpose of analyzing crustal deformation at two times, it is desirable to use a pair of SAR data that satisfies the following conditions:                An observation date/time of master data is near an expected date/time of master data; and        An observation date/time of slave data is near an expected date/time of slave data.        
PTL 1 describes a method of selecting a pair of SAR data, based on a length of a baseline connecting between two satellites that are arbitrarily selected from a plurality of satellites, for the purpose of extracting altitude information from the pair of SAR data.